This invention relates generally to radar systems, and more particularly to a segmented cylindrical corner reflector as a radar target for calibrating the orthogonal polarizations of a millimeter wave radar.
The increasing utilization of millimeter wave bands for radar applications has led to a need for reflectivity measurements at the corresponding wavelengths. As the frequency increases, smaller scatterers and resonance effects become more important making it very difficult to predict the behavior of the reflective properties of scatterers at millimeter wavelengths. The problems encountered in performing measurements at millimeter waves as opposed to microwaves requires that different techniques be used to resolve the problems.
The polarizations used in reflectivity measurements are typically vertical and horizontal, or left and right circular, although any two orthogonal polarizations can be used. The polarization scattering matrix is a two-by-two complex matrix, with each element of the matrix representing the amplitude and phase of reflection from a target for one of four orthogonal polarization states.
Calibration is especially important at millimeter wavelengths as compared to microwave wavelengths because of the greater effects of variations in measurement equipment and in the environment at the shorter wavelengths. Calibration procedures can be classified as amplitude calibration, phase calibration, and polarization calibration. Amplitude calibration typically involves comparison of a target to be measured with a standard target of known radar cross section (RCS) properties. Phase calibration is necessary for coherent systems to provide phase linearity and stability.
Polarization calibration involves the measurement of the polarization isolation of the system and the use of calibration targets to calibrate the components of the polarization matrix. Polarization isolation can be measured for a dual-polarized radar by transmitting one polarization and receiving the return from a non-polarizing target with the orthogonal polarization.
Radar targets are passive reflectors which have a reflected signal distribution similar to the patterns of an antenna. The basic problem in the design of radar targets is that of maximizing the target returns. The types of radar targets include sphere, cylinder, flat plate, diplane (dihedral corner), triangular trihedral, square trihedral, circular trihedral, and top hat.
The sphere is an easy target to manufacture and has an RCS which is independent of frequency. Its primary disadvantage is a very low RCS for a given size sphere. The cylinder has a narrow angle of return in the plane along its axis and a broad region of return in the plane along the radius. The cylinder is used for calibrating RCS ranges since it can be rotated in azimuth to find the specular return, while orienting the broad radial lobe in the vertical direction.
There are three types of trihedrals, i.e., triangular, square, and circular. The trihedral has wide lobes in both planes and exhibits a relatively large RCS. The triangular trihedral has the widest lobe. The flat plate has the largest RCS for its area of any of the targets but has a narrow lobe in both the vertical and horizontal planes. The plate is hard to align, but when calibration is performed near the ground, its narrow vertical lobe rejects multipath signals.
The diplane (dihedral corner) is the reflector normally used for calibrating orthogonal polarizations. It has a broad beam in the plane perpendicular to the seam and a very narrow beam in the plane along the seam. Its primary disadvantage is in properly aiming it towards the radar because of its narrow beam and the fact that it is often rotated from vertical to obtain orthogonally polarized returns. The top hat reflector has the polarization properties of a diplane while possessing a broad lobe in both the vertical and horizontal planes. Its major disadvantage is its small RCS for a given physical size.
Calibration of orthogonal linear polarizations is based on a unique property of the diplane. If a linearly polarized wave is incident at an angle .sigma. relative to the seam between the faces, the reflected wave is also linearly polarized, at an equal, but opposite, angle to the seam. If a dihedral is rotated about an axis parallel to the radar line of sight, the reflected polarization will rotate in the opposite direction at twice the rate. A rotation of 45.degree. to an incident linear polarized wave will return an orthogonal wave.
Circular polarization requires the use of two different types of targets for calibration: an odd-bounce target and an even-bounce target. The cylinder, sphere, trihedral, and flat plate are odd-bounce targets which exhibit an odd number of bounces for incident radiation. An odd-bounce reflector always returns the opposite sense circular polarization because of an odd number of 180.degree. phase reversals at each bounce. As an example, when a left hand circularly polarized wave is reflected from a trihedral, it becomes a right hand circularly polarized wave. A trihedral is commonly used to calibrate odd-bounce circular polarized signals.
The diplane and top hat are even-bounce targets. An even-bounce reflector always returns the same sense circular polarization as the incident wave. For example, a left hand circularly polarized wave is reflected as a left hand circular wave. A diplane is commonly used to calibrate even-bounce circular polarized signals.
There is a need in the art for a radar target for calibrating orthogonal polarizations which has a relatively large RCS and simultaneously a wider lobe in the plane parallel to the seam than does a diplane.